Exhaust emission control device of internal combustion engine

ABSTRACT

An exhaust gas purifying system reliably detects an amount of accumulated particulates, lengthens forced regeneration intervals, and improves fuel efficiency. The exhaust gas purifying system comprises: a particulate filter  22  disposed in an exhaust system of an internal combustion engine  2  and collecting particulates from exhaust gas, and an NO 2  generating unit  21  upstream of or in the particulate filter  22 ; a discharged particulate amount calculating unit A 1  calculates an amount Me of discharged particulates on the basis of an excess air ratio λ; a burnt particulate amount calculating unit A 2  calculates an amount Mb of burnt particulates on the basis of the temperature of exhaust gas in front of the particulate filter or the temperature of the particulate filter; and a particulate accumulation amount calculating unit calculates an amount Ma of accumulated particulates on the basis of the amount Me of discharged particulates or the amount Mb of burnt particulates.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an exhaust gas purifying system configured tocollect carbon particles and so on from exhaust gas of an internalcombustion engine, and more particularly to an exhaust gas purifyingsystem which oxidizes and burns carbon particles trapped on a filterusing nitrogen dioxide (NO₂) generated by an oxidation catalyst.

2. Description of the Related Art

Particulates composed of carbon particles and so on get mixed in exhaustgas of an internal combustion engine, e.g. a diesel engine. Aparticulate filter is installed in an exhaust passage in order to trapparticulates and prevent them from being discharged into the air. Whenmore particulates are accumulated on the particulate filter, they shouldbe burnt in order to regenerate the particulate filter.

In order to overcome the foregoing problem, forced regenerating unitsare used, which heat the particulate filter and burn particulates whenan amount of accumulated particulates exceeds a regeneration referencepoint. Specifically, the amount of accumulated particulates is detectedon the basis of relationship between the flow of exhaust gas andpressure loss of the particulate filter. For instance, some forcedregenerating unit injects fuel to a fuel supply system during anexpansion stroke or an exhaust stroke after the main fuel injection, andforcibly raises the temperature of exhaust gas. In another example, anelectric heater or a gas oil burner is operated to forcibly heat exhaustgas.

The foregoing forced regenerating units tend to reduce the fuelefficiency since particulate filters should be kept hot. In order toovercome this problem, it is necessary to precisely detect forcedregeneration timings and lengthen forced regeneration intervals.

Usually, particulates can be oxidized at approximately 600° C. There isa continuous regeneration type filter in which particulates can be burntat a low temperature of approximately 250° C. This enables particulatesto be burnt in a wide temperature range and promotes regeneration ofparticulate filters.

In the continuous regeneration type filter, an oxidation catalyst isdisposed upstream of a particulate filter in an exhaust passage. Theoxidation catalyst oxidizes nitric monoxide (NO) and generates nitrogendioxide (NO₂) as expressed by the formula (1).2NO+O₂→2NO₂  (1)

Nitrogen dioxide (NO₂) is very active, and promotes the reactionexpressed by formulas (2) and (3) when it comes into contact withparticulates trapped on the particulate filter, thereby regenerating theparticulate filter.NO₂+C→NO+CO  (2)NO₂+CO→NO+CO₂  (3)

However, the continuous regeneration type filter which can burnparticulates at the low temperature fails to raise the temperature ofexhaust gas when a vehicle keeps on cruising through town at a low load.In such a case, particulates easily accumulate on the particulatefilter, and should be forcibly burnt in order to regenerate theparticulate filter.

Therefore, the foregoing continuous regeneration type filter usuallyincludes a forced regeneration unit, which forcibly heats exhaust gas onthe particulate filter, and burns particulates when the amount ofaccumulated particulates is detected to be above the regenerationreference point mentioned above. In this case, the forced regeneratingunit injects fuel to a fuel supply system during the expansion stroke orexhaust stroke after the main fuel injection, and forcibly heats exhaustgas.

For instance, the assignee of this application has proposed a method ofeasily estimating an amount of particulates accumulated on a filter onthe basis of an exhaust gas temperature frequency (i.e. at which anexhaust gas temperature is equal to or higher than a predeterminedvalue) as disclosed in Japanese Patent Application No. 2001-144,501(called the “cited reference 1”). Further, Japanese Patent Laid-OpenPublication No. 2002-276,422 (called the “cited reference 2”) disclosesa continuous regeneration type DPF (diesel particulate filter) in whichan oxidation catalyst, a particulate filter and a NOx catalyst arearranged upstream of an exhaust passage in order to operate an engine byincreasing an air-to-fuel ratio during the regeneration of theparticulate filter.

In either the continuous regeneration type filter or a simpleparticulate filter, particulates are burnt when the amount ofaccumulated particulates exceeds a regeneration reference point. If theamount of accumulated particulates is not precisely detected, e.g. ifthe accumulated amount is recognized to be excessive, regenerationintervals may be shortened, which reduces fuel efficiency. On thecontrary, if the accumulated amount is determined to be small,particulates excessively accumulate on the filter, and may damage thefilter when burnt. Therefore, it is necessary to precisely detect forcedregeneration timings and to lengthen forced regeneration intervals.

The foregoing method allows detection of the amount of accumulatedparticulates on the basis of the relationship between the flow ofexhaust gas and pressure loss of the filter. However, there is a strongdemand for a method of precisely estimating an amount of accumulatedparticulates. Especially, in the case of the continuous regenerationtype filter, particulates tend to be partially burnt, which would leadto non-uniform accumulation of particulates, and further adverselyaffect relationship between the flow rate of exhaust gas, the pressureloss and the amount of accumulated particulates.

The continuous regeneration type filter (of the cited reference 1) ispreferably to be improved. This is because the amount of burntparticulates can be estimated while the amount of dischargedparticulates cannot be accurately estimated, which would adverselyaffect precise detection of the amount of accumulated particulates. Inthe continuous regeneration type filter (of the cited reference 2), thetiming to regenerate the particulate filter is not determined on thebasis of the amount of accumulated particulates, but the particulatefilter is regenerated only by increasing the air-to-fuel ratio, whichtends to reduce fuel efficiency.

The present invention is intended to provide an exhaust gas purifyingsystem for an internal combustion engine which can precisely detect aforced regeneration timing, lengthen regeneration intervals, and preventreduction of fuel efficiency.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided anexhaust gas purifying system for an internal combustion engine,comprising: an exhaust-after-treatment device disposed in an exhaustsystem of the internal combustion engine, and including a particulatefilter configured to collect particulates from exhaust gas, and an NO₂generating unit upstream of or in the particulate filter; a dischargedparticulate amount calculating unit configured to calculate an amount ofdischarged particulates on the basis of an excess air ratio; a burntparticulate amount calculating unit configured to calculate an amount ofburnt particulates on the basis of a temperature of exhaust gas in frontof the particulate filter or a temperature of the particulate filter;and a particulate accumulation amount calculating unit configured tocalculate an amount of accumulated particulates on the basis of thecalculated amount of discharged particulates or the calculated amount ofburnt particulates.

The amount of accumulated particulates can be precisely detected bycalculating the amount of burnt particulates on the basis of the exhaustgas temperature or the filter temperature, and by calculating the amountof discharged particulates on the basis of the excess air ratio. This iseffective in properly setting up forced regeneration intervals.

The exhaust gas purifying system preferably includes a forcedregeneration system which raises the temperature of exhaust gas byinjecting additional fuel in an expansion or exhaust stroke after themain fuel injection or provides hydrocarbon HC to a catalyst or thefilter in order to burn particulates on the filter, when the amount ofaccumulated particulates exceeds a predetermined value. In such a case,a light oil burner or an electric heater is usable for the forcedregeneration.

In accordance with a second aspect of the invention, there is providedan exhaust gas purifying system for an internal combustion engine,comprising: an exhaust-after-treatment device disposed in an exhaustsystem of the internal combustion engine, and including a particulatefilter configured to collect particulates from exhaust gas, and an NO₂generating unit upstream of or in the particulate filter; an excess airratio deviation frequency calculating unit configured to calculate afrequency at which an excess air ratio is equal to or less than apredetermined value during the operation of the internal combustionengine; a discharged particulate amount calculating unit configured tocalculate an amount of discharged particulates on the basis of theexcess air ratio frequency at which the excess air ratio is equal to orless than the predetermined value; a filter temperature frequencycalculating unit configured to calculate a filter frequency at which thetemperature of exhaust gas in front of the particulate filter or thetemperature of the particulate filter is equal to or higher than apredetermined value; a burnt particulate amount calculating unitcalculating an amount of burnt particulates on the basis of thefrequency at which the temperature of exhaust gas or the temperature ofthe particulate filter is equal to or high than the predetermined value;and a particulate accumulation amount calculating unit configured tocalculate an amount of particulates accumulated on the particulatefilter on the basis of the calculated amount of discharged particulatesand the calculated amount of burnt particulates.

The amount of burnt particulates is calculated on the basis of theparticulate burning velocity which depends upon the exhaust gastemperature or upon the filter frequency at which the filter temperatureis equal to or higher than the predetermined value. Further, the amountof discharged particulates is calculated on the basis of the excess airratio frequency at which the excess air ratio is equal or less than thepredetermined value. Therefore, the amount of accumulated particulatescan be precisely detected, which is effective in properly setting up theforced regeneration intervals.

The discharged particulate amount calculating unit calculates an amountof particulates discharged in a given time period during which theexcess air ratio is equal to or less than the predetermined value. Theburnt particulate amount calculating unit includes a burning velocitycalculating section which calculates a velocity for burning particulateson the particulate filter on the basis of the filter temperaturefrequency at which the temperature of exhaust gas in front of theparticulate filter or the temperature of the particulate filter is equalto or higher than the predetermined value, and derives an amount ofparticulates burnt in the given time period on the basis of theparticulate burning velocity in the given time period and the amount ofparticulates accumulated in the given time period. The particulateaccumulation amount calculating unit calculates an amount of currentlyaccumulated particulates on the basis of the amount of previouslyaccumulated particulates, the amount of particulates discharged duringthe given time period, and the amount of burnt particulates in the giventime period.

The amount of particulates burnt in the given time period is calculatedon the basis of the particulate burning velocity in the given timeperiod and the amount of previously accumulated particulates. The amountof particulates discharged in the given time period is calculated on thebasis of the excess air ratio frequency at which the excess air ratio isequal to or less than the predetermined value in the given time period.Further, the amount of currently accumulated particulates is calculatedon the basis of the amount of previously accumulated particulates, theamount of particulates accumulated in the given time period, and theamount of particulates burnt in the given time period. Therefore, theamount of currently accumulated particulates can be precisely detected,which is effective in properly setting up the forced regenerationintervals.

Alternatively, the particulate accumulation amount calculating unit maycalculate the excess air ratio frequency in a given time period byweight-averaging the excess air ratio frequency, at which the excess airratio is equal to or less than the predetermined value, through the useof a weighting factor wf. In this case, the factor wf is assumed to be0.5. The nearer the weighting factor wf becomes 1, the less influence ofthe previous excess air ratio frequency. The use of the excess air ratiofrequency calculated using the weighting factor wf is effective inadjusting variations of data caused by disturbance. Therefore, theamount of discharged particulates can be precisely detected.

Still further, the discharged particulate amount calculating unit maycalculate an excess air ratio frequency β_(i) in a given time periodwhen the excess air ratio is equal to or less than the predeterminedvalue, using the following formula.β_(i)=(xi+β _(i−1)×(i−1))/iwhere: xi (i.e. an i-th determination value) is 1 when the excess airratio is equal to or less than the predetermined value, and xi is 0 whenthe excess air ratio is above the predetermined value; β_(i) is an i-thexcess air ratio frequency; β_(i−1) denotes an excess air ratiofrequency prior to the i-th excess air ratio frequency.

The filter temperature frequency at which the filter temperature isequal to or more than the predetermined value may be calculatedsimilarly. This is effective in detecting the amount of dischargedparticles.

The given time period may be the unit time, a time period in which apredetermined amount of fuel is consumed, or a time period for a vehicleto run a certain distance. In this case, the foregoing effects can beaccomplished.

The calculation of the amount of discharged particulates includes:downloading data on an amount of intake air and data on an amount ofinjected fuel: calculating an excess air ratio λ in the given timeperiod Δt on the basis of the amount of intake air and the amount ofinjected fuel; calculating an excess air ratio frequency γΔt in, inwhich the excess air ratio λ is the predetermined value or less in thegiven time period Δt, on the basis of the excess air ratio λ; andcalculating the amount of discharged particulates MeΔt {=f(λΔt}. Theforegoing procedures are sequentially executed.

The amount of particulates discharged in the given time interval can beprecisely calculated. This promotes precise detection of the amount ofcurrently accumulated particulates, and establishes proper forcedregeneration intervals.

Further, the calculation of the amount of burnt particulates includes:downloading a catalyst temperature gt; calculating a filter gastemperature frequency βΔt in a given time period Δt on the basis of thecatalyst temperature gt; correcting the filter temperature frequency βΔtusing a correction factor K which depends upon an index NOx/Sootrepresenting that components of exhaust gas are suitable for burningparticulates; calculating a burning velocity coefficient αΔt {=f(βΔt)}for the given time period Δt; and calculating an amount MbΔt{=αΔt×PM_(i−1)} of burnt particulates on the basis of an amount Ma_(i−1)of previously accumulated particulates and the burning velocitycoefficient αΔt, the foregoing procedures being conducted in the namedorder.

The amount of particulates burnt in the given time interval can beprecisely calculated. This promotes precise detection of the amount ofcurrently accumulated particulates, and establishes proper forcedregeneration intervals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic arrangement of a first embodiment of an exhaustgas purifying system for an internal combustion engine according to thepresent invention.

FIG. 2 is a block diagram showing functions of the exhaust gas purifyingsystem of FIG. 1.

FIG. 3A shows a map for estimating an amount of discharged particulateson the basis of an excess air ratio.

FIG. 3B shows a map for estimating a particulate burning velocity on thebasis of a filter temperature frequency at which a filter or exhaust gastemperature is equal to or higher than a predetermined value.

FIG. 3C shows a map for easy estimation of a burning velocitycoefficient on the basis of the filter temperature frequency, theburning velocity coefficient being used at the time of forcedregeneration of a filter.

FIG. 4A shows a map for explaining time-dependent variations of anexcess air ratio frequency at which the excess air ratio is equal to orless than a predetermined value, used for forced regeneration of thefilter.

FIG. 4B shows a waveform of a moving weight average of the excess airratio frequency.

FIG. 5A shows a map for estimating NOx/Soot on the basis of a fuelinjection amount and an engine speed.

FIG. 5B shows a map for setting up a correction factor K on the basis ofNOx/Soot.

FIG. 6 is a flow chart of a forced regeneration routine of the exhaustgas purifying system.

FIG. 7 is a chart for explaining post fuel injection executed in step s5of the forced regeneration routine of FIG. 6.

FIG. 8 is similar to FIG. 2, but showing functions of an exhaust gaspurifying system according to a second embodiment of the invention.

FIG. 9A is a flow chart of a forced regeneration routine of the exhaustgas purifying system of FIG. 8, especially showing a routine fordetecting forced regeneration timing.

FIG. 9B is a flow chart for calculating an amount of particulatesdischarged during a given time period.

FIG. 9C is a flow chart for calculating an amount of particulates burntduring a given time interval.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described with reference to a first embodimentshown in FIG. 1 to FIG. 7.

Referring to FIG. 1, an exhaust gas purifying system 1 is installed in adiesel engine 2 (called the “engine 2” hereinafter). The engine 2includes an exhaust passage R extending from a combustion chamber 3. Theexhaust passage R connects to an exhaust manifold 4, an exhaust pipe 5,an exhaust-after-treatment device 6 interposed in the exhaust pipe 5 anda silencer (not shown). The engine 2 is an in-line four-cylinder engine.Each cylinder is provided with an injector 8, to which a fuel supplyingsection 9, and a fuel injecting section 11 are connected. The fuelinjecting section 9 injects fuel to the combustion chamber 3 via theinjector 8. An engine control unit ECU12 controls the injectors 8 andmembers connected thereto.

The fuel supply section 9 stabilizes high pressure fuel from a highpressure fuel pump 13 under the control of a fuel pressure regulator 121in the engine control unit ECU12, and introduces the stabilized fuel toa common rail 15. The fuel is then supplied to each injector 8 via afuel pipe 16 branching from the common rail 15. In the injector 8, amagnetic valve 17 is connected to an injection controller 122, whichprovides the magnetic valve 17 with output signals representative of anamount of fuel to be injected and an injection timing, therebycontrolling the operation of the injectors 8.

The injection controller 122 calculates the amount of fuel to beinjected and injection timing in accordance with an engine speed Ne andan amount θ a of accelerator pedal depression. Thereafter, the injectioncontroller 122 outputs the calculated results to an injector driver 10,which transfers them to the magnetic valve 17 of the fuel injectingsection 11.

The exhaust-after-treatment device 6 is housed in a metal casing 18. Anoxide catalyst 21 and a diesel particulate filter 22 (called the “filter22”) are placed in series in a bulging part 181 of the casing 18 via asupport 19 made of asbestos or a bulky metal wire netting.

The oxide catalyst 21 is housed in a catalyst holder 211, in which aplurality of exhaust passages r1 are formed. The exhaust passages r1 areopen at their opposite ends and enable exhaust gas to pass therethrough.The catalyst holder 211 is made of ceramics and has a monolithichoneycomb structure. The exhaust passages r1 are in parallel with oneanother in the catalyst holder 211, and hold the oxide catalyst 21therein.

The oxide catalyst 21 oxidizes nitric monoxide NO in exhaust gas fromthe engine 2 using oxygen O₂, and generates very active nitrogen dioxideNO₂, i.e. the oxide catalyst 21 should promote the generation of NO₂ asexpressed in the formula (1). In order to meet this requirement, aplatinum group oxide catalyst is employed in the invention.

The filter 22 is made of ceramics, e.g. cordierite mainly containing Mg,Al and Si, and has a honeycomb structure in order to constitute aplurality of upstream and downstream exhaust passages r2 (r2-1 and r2-2)which are aligned toward the exhaust pipe 5 and are in parallel with oneanother. Adjacent exhaust passages r2 are alternately opened or closedat their front or rear ends 23. Exhaust gas is introduced into eachupstream exhaust passage r2-1, passes through a wall b defining theexhaust passage r2-1, reaches each downstream exhaust passage r2-2having an open end, and is discharged into the air. In this process,particulates are filtered from exhaust gas.

The engine control unit ECU12 is connected to an air flow sensor 7detecting an amount Qa of intake air, an accelerator opening sensor 24detecting an opening angle θ a of an accelerator pedal of the engine 2,a crank angle sensor 25 detecting crank angle data Δθ, an exhausttemperature sensor 26 detecting the temperature gt of exhaust gas, awater temperature sensor 27 detecting the water temperature wt, anatmospheric pressure sensor 28, and an idle switch 29 outputting an idlesignal ID. The crank angle data Δθ is used for the engine control unitECU12 to derive an engine speed Ne and to control a fuel injectiontiming (to be described later).

Further, the engine control unit ECU12 is provided, in its output andinput circuits, with a plurality of ports in order to download a varietyof signals from the accelerator pedal opening sensor 24, crank anglesensor 25, exhaust temperature sensor 26, water temperature sensor 27,atmospheric pressure sensor 28 and so on. Still further, the enginecontrol unit ECU12 includes a fuel pressure controller 121, an injectioncontroller 122, and a forced regeneration control section, whichincludes a unit A1 calculating an amount of discharged particulates, aunit A2 calculating an amount of burnt particulates, and a unit A3calculating an amount of accumulated particles (refer to FIG. 2), all ofwhich are well-known.

The unit A1 calculates the amount Me of discharged particulates on thebasis of an excess air ratio λ, and using a map m1 (FIG. 3A).

The unit A2 calculates an amount Mb of burnt particulates on the basisof the temperature gt of exhaust gas in front of the filter 22 or thetemperature of the filter 22. The temperature of the filter 22 isconsidered to be equal to the exhaust gas temperature, and is alsorepresented by “gt”.

The unit A3 calculates an amount Ma of particulates accumulated on thefilter 22 on the basis of the amount Me of discharged particulates andthe amount Mb of burnt particulates.

When the engine 2 provided with the exhaust gas purifying system 1 isstarted, the engine control unit ECU12 checks, in a main routine (notshown), whether or not the signals from the foregoing sensors arenormal. When they are normal, the engine 2 will be activated.

During the operation of the engine 2, exhaust gas flows into a pluralityof exhaust passages r1 of the catalyst holder 211, so that nitrogenmonoxide (NO) in exhaust gas is oxidized and changes into very activenitrogen dioxide (NO₂), as expressed by the formula (1). Exhaust gaswith nitrogen dioxide NO₂ is guided to the filter 22. In the filter 22,exhaust gas passes through the walls b defining the exhaust passagesr2-1, reaches the exhaust passages r2-2, and is discharged into the air.Particulates are trapped in the filter 22 while exhaust gas passesthrough the walls b.

In this state, the forced regeneration control is executed in the mainroutine shown in FIG. 6.

During the forced regeneration control, the following are calculated:the amount Me of discharged particulates in step s1; the amount Mb ofburnt particulates in step s2; and the amount Ma of accumulatedparticulates in step s3. When the amount Ma of accumulated particulatesis greater than a predetermined threshold Maε in step s4, the controlprocess is advanced to step s5, where the forced regeneration controlwill be performed in order to forcibly regenerate the filter 22 (e.g.post-injection control will be carried out for a predetermined timeperiod).

The procedures shown in solid line squares in FIG. 2 are executed duringthe calculation of the amount Me of discharged particulates in step s1.The unit A1 downloads a latest amount Qa of intake air and a latestamount Qf of injected fuel, and calculates an excess air ratio λ{=Qa/(Qf×14.7)} using an excess air ratio calculator a1. The excess airratio calculator a1 also calculates an amount Me of particulatesdischarged in response to the excess air ratio λ, using the map m1showing the amount of discharged particulates. The map m1 is preparedbeforehand, and shows that as the excess air ratio λ is lowered, themore abruptly the amount Me of discharged particulates increases.

In step s2, the unit A2 downloads the filter temperature gt and operatesa simplified calculator b0 in order to calculate the amount Mb of burntparticulates. Refer to FIG. 2.

Specifically, the simplified calculator b0 calculates a burning velocitycoefficient α corresponding to the filter temperature gt. The map m0shown in FIG. 3( c) is used for this purpose. The map m0 shows that theburning velocity coefficient α increases in response to the filtertemperature gt.

A calculator b4 calculates the amount Mb of burnt particulates on thebasis of the formula (b).Mb=α×PM×t  (b)where PM denotes an amount of particulates accumulated at a time ofmeasurement and corresponds to an amount of previously accumulatedparticulates, and “t” denotes a unit time.

In step s3, the unit A3 calculates the amount Ma of accumulatedparticulates, as shown in FIG. 2, using the following formula (c).Ma=Me−Mb  (c)where Me denotes the amount of particulates discharged per unit time t.

The amount of Ma_(i) of currently accumulated particulates is added tothe an amount Ma_(i−1) of particulates previously accumulated during apredetermined time period mt, so that a total amount Maptm ofparticulate is derived.

In step s4, it is checked whether or not the total amount Maptm is abovethe predetermined threshold Maε. The calculations in steps s1 to s4 arerepeated until the amount Maptm is above the predetermined thresholdMaε. The threshold Maε is determined in order to prevent the filter 22from being overheated and damaged when particulates thereon arecontinuously burnt.

When Maptm>Maε, post-fuel injection is conducted for a predeterminedtime period in step s5 in order to forcibly heat and regenerate thefilter 22. Specifically, as shown in FIG. 7, not only an amount INJn offuel injected (for an injection period Bm) in the main injection J1 butalso a fuel injection timing t1 are calculated in accordance with acurrent state of the engine 2. Further, a post injection amount INJp offuel to be post-injected (for an injection period Bs) is set to a fixedvalue at a fuel injection timing t2 after the main fuel injection.

The following data are sent to the fuel injection driver 10: an outputDinj representing the fuel injection amount INJn and the fuel injectiontiming t1; and an output D'inj representing the post injection amountINJp and the post fuel injection timing t2. Then, the control processreturns to the main routine. Thereafter, the fuel injection driver 10counts unit crank angles Δθ for a predetermined number of times from areference timing (TDC) till a fuel injection timing θr, carries out themain and post fuel injections J1 and J2. Exhaust gas is heated,hydrocarbon HC is burnt on the oxide catalyst a, the temperature gt ofthe filter 22 is quickly raised, and particulates are burnt in a hotatmosphere for a time period which depends upon the amount ofaccumulated particulates. As a result, the filter 22 is reliablyregenerated in the forced regeneration process.

The amount Ma of accumulated particulates can be accurately detected bycalculating the amount Me of discharged particulates on the basis of theexcess air ratio λ and by calculating the amount Mb of burntparticulates on the basis of the filter temperature gt. Further, thetime intervals between the previous and current regenerations can beproperly set, which is effective in maintaining the fuel efficiency in aproper range.

The filter 22 is forcibly heated by the post fuel injection J2 in theexpansion stroke after the main fuel injection J1, so that it is notnecessary to provide any special external heat source for the forcedregeneration. This is effective in simplifying the exhaust gas purifyingsystem. Alternatively, a light oil burner or an electric heater (notshown in FIG. 6) may be provided in the exhaust-after-treatment deviceas a forced regeneration unit, and be activated in order to promote theregeneration of the filter 22 in step s5. In such a case, the fuelcontrol system may be controlled in a simple manner.

An exhaust gas purifying system will be further described with referenceto a second embodiment shown in FIGS. 8 and 9. The exhaust gas purifyingsystem is configured similarly to that of the first embodiment.

Referring to FIG. 8, a unit A1′ calculates the amount of dischargedparticulates, a unit A2′ calculates the amount of burnt particulates,and a unit A3′ calculates the amount of accumulated particulates.

First of all, the unit A1′ calculates the excess air ratioλ{=Qa/(Qf×14.7)} using an excess air ratio calculator a1′. A sectiona2-1′ calculates an excess air ratio frequency γ at which the excess airratio λ is equal to or less than the predetermined value in a given timeinterval Δt. Referring to FIG. 4A, when the excess air ratio λ is equalto or less than the predetermined value (e.g. 1.2), a determinationvalue x is set to 1. On the contrary, when the excess air ratio λ isabove than the predetermined value, the determination value x is set to0. Based on the foregoing determination, the excess air ratio frequencyγ is calculated using the moving weight average formula (g).γi=(γ_(i−1)×(i−1)+γi)/i  (g)where γi denotes an i-th excess air ratio frequency, and γ_(i−1) denotesan excess air ratio frequency prior to the excess air ratio frequencyγ_(i).

Referring to FIG. 4B, the excess air ratio frequency γ_(i) at the end ofcalculation in the time period Δt is assumed to be γΔt.

In this case, no large memory is necessary, and the excess air ratiofrequency γ can be observed in a chorological order.

The excess air ratio frequency γi may be derived by using the formula(h).γi=Δ _(i−1) ×wf+xi×(1−wf)  (h)where wf denotes a weighting factor, and xi denotes a currentdetermination value. The weighting factor wf is assumed to be 0.5. Thenearer the weighting factor wf becomes 1, the less influence of theprevious excess air ratio frequency γ_(i−1). The use of the excess airratio frequency γ calculated using the weighting factor wf is effectivein adjusting variations of data caused by disturbance. Therefore, theamount Me of discharged particulates can be precisely detected.

A section a2-2′ calculates an amount MeΔt of particulates dischargedduring the time period Δt, using the formula (i).MeΔt=f(γΔt)  (i)

Further, the amount Me of discharged particulates may be derived bymultiplying the excess air ratio frequency λΔt (in the time period Δt)by a predetermined coefficient C. The coefficient C is experimentallydetermined. Still further, the amount Me may be derived using a map inwhich the amount Me of discharged particulates is depicted on the basisof the excess air ratio frequency γΔt, in place of using the formula(i).

For instance, when the excess air ratio shown in FIG. 3A is substitutedby the excess air ratio frequency γ, the amount Me of dischargedparticulates is depicted by a curve opposite to that of FIG. 3A, i.e.the larger the excess air ratio frequency γ, the larger the amount Me(or the higher a particulate discharging velocity θ).

A unit A2′ in FIG. 8 calculates an amount Mb of burnt particulates.Specifically, the unit A2′ downloads the filter temperatures gt per unittime using a section b1 for calculating a filter temperature frequency,totals the filter temperatures gt, and derives a filter temperaturefrequency βΔt in the time period Δt.

In the foregoing case, if the filter temperature gt is downloaded eachunit time t, a large memory is required, which is not effective in viewof cost. In order to overcome this problem, the filter temperaturefrequency βΔt may be calculated using the formula (j).β_(i)=(β_(i)+β_(i−1)×(i−1)/i  (j)where β_(i) denotes an i-th filter temperature frequency, and β_(i−1)denotes a previous filter temperature frequency.

In this case, the filter temperature frequency β can be observed inchorological order without using a large memory.

A filter temperature frequency corrector b2 corrects the filtertemperature frequency βΔt (in the time period Δt) using a correctioncoefficient in accordance with the NOx/Soot.

Particulates can be usually burnt at a lowest temperature ofapproximately 600° C. However, the use of the oxide catalyst 21 andoxidative reaction with NO₂ can reduce the lowest temperature to 250° C.Generation of NO₂ depends upon an amount of NOx in exhaust gas, i.e. themore NOx, the more NO₂. Therefore, particulates can be reliably burnt atapproximately 250° C. Conversely, the less the NOx, the less NO₂. Thismeans that particulates may not be burnt reliably at approximately 250°C. In other words, burning efficiency of particulates depends upon theamount of NOx in exhaust gas, and more particularly upon the NOx/Sootserving as an index which indicates whether or not exhaust gas containscomponents suitable to burn particulates.

For the foregoing reasons, the filter temperature frequency corrector b2sets up the NOx/Soot in response to the engine speed Ne and the fuelinjecting amount Qf (corresponding to torque) and using the NOx/Soot mapm4 in FIG. 5A and a correction coefficient map m5 in FIG. 5B, andcalculates a correction coefficient Ka on the basis of the NOx/Soot. Forinstance, if the NOx/Soot is 25 or larger, the correction coefficient Kgradually exceeds 1. If the NOx/Soot is less than 25, the correctioncoefficient K gradually becomes smaller than 1 in response to thereduction of the NOx/Soot. Further, the correction coefficient K is setto be a steady value (<1) when the NOx/Soot is less than 16. Further,the filter temperature frequency corrector b2 multiplies the correctioncoefficient K with the temperature frequency β, thereby correcting thecoefficient K.

A burning velocity calculator b3 calculates a particulate burningvelocity coefficient αΔt in the time period Δt using the formula (k).αΔt=f(βΔt)  (k)

The particulate burning velocity coefficient αΔt may be derived usingthe map m2 shown in FIG. 3B, in place of the formula (k).

Specifically, the larger the filter temperature frequency βΔt in thegiven time period, the larger the particulate burning velocitycoefficient αΔt.

A burnt particulate amount calculator b4″ calculates an amount MbΔt ofparticulates burnt in the time period Δt using the formula (1).MbΔt=αΔt*PM_(i−1)  (1)where PM_(i−1) represents the amount of previously accumulatedparticulates, which is calculated by the unit A3′ calculating an amountof accumulated particulates as will be described later.

Alternatively, the amount MbΔt may be derived using a map showing therelationship between the particulate burning velocity αΔt and the amountMb of burnt particulates.

The larger the particulate burning velocity coefficient αΔt, the morethe amount MbΔt.

The unit A3′ calculates an amount PM_(i) of currently accumulatedparticulates using the formula (m).PM _(i) =PM _(i−1)+(MeΔt−MbΔt)×Δt  (m)

In the foregoing embodiment, the burnt particulate amount calculator b4′of the unit A2′ calculates the burnt particulate amount MbΔt.Alternatively, the amount PM_(i) of currently accumulated particulatesmay be calculated by the unit A3′ using the formula (n) when the unitA2′ is replaced by a unit A2″ including the burning velocity calculatorb3.PM _(i) =PM _(i−1)+(MeΔt−αΔt×PM _(i−1))×Δt  (n)

A forced regeneration routine will be described with reference to FIG.9A to FIG. 9C. Specifically, FIG. 9A shows a forced regeneration timingdetecting routine.

The amount MeΔt of particulates discharged in the time period Δt iscalculated in step s10, and the amount MbΔt of burnt particulates in thetime period Δt is calculated in step s20.

A routine shown in FIG. 9B is used for this purpose. In step s11, anintake air amount Qa and a fuel injection amount Qf are downloaded. Instep s12, the excess air ratio λ in the time period Δt is calculated onthe basis of the downloaded data. In step s13, the excess air ratiofrequency γ is calculated by the excess air ratio frequency calculatora2-1′ shown in FIG. 8. Finally, the amount MaΔt {=f(γΔt)} is calculatedin step s14.

The amount MbΔt of particulates burnt in the given time period Δt iscalculated in a routine shown in FIG. 9C.

The catalyst temperature gt is downloaded in step s21, and the filtertemperature frequency βΔt is calculated on the basis of the catalysttemperature gt in step s22, and is corrected using a correctioncoefficient depending K upon the NOx/Soot. In step s23, the particulateburning velocity αΔt{=f(βΔt)} is calculated using the filter temperaturefrequency βΔt. Finally, the amount MbΔt {=αΔt×PM_(i−1)} is calculated instep s24.

Following the calculations of MeΔt and MbΔt in steps s10 and s20, theamount PM_(i) of currently accumulated particulates is calculated usingPM_(i−1), MeΔt and MbΔt in step 30. Refer to FIG. 9A.

When the amount PM_(i) is detected to be equal to or larger than thepredetermined value in step s40, the forced regeneration is executed instep s50 in order to forcibly heat the filter 22. For this purpose, apredetermined amount of fuel is post-injected at an appropriate timingfor a necessary time interval after the main fuel injection.

Therefore, exhaust gas is heated, so that the filter temperature gt isquickly raised, and particulates are adequately burnt in a hotatmosphere. This allows reliable regeneration of the filter 22.

The amount PM_(i) of accumulated particulates can be accurately detectedby calculating the amount Me of particulates discharged in the timeperiod Δt and the amount Mb of particulates burnt in the time period Δt.Therefore, forced regeneration intervals can be properly set up andlengthened, which is effective in preventing the reduction of fuelefficiency.

Further, the burnt particulate amount calculating unit A2′ may derivethe filter temperature frequency βc, where a filter temperature gt is250° C. or higher for the time period Δt, or may derive an average ofthe filter temperature frequency βc in the time period Δt.

The foregoing alternatives are as effective as the forced regenerationprocedure shown in FIGS. 9A to 9C. The total amount of accumulatedparticulates can be accurately detected, which is effective keeping theforced regeneration interval in a proper range.

In the foregoing description, the filter has the honeycomb structure.Alternatively, the filter may be in shape of a wire mesh or have athree-dimensional structure.

INDUSTRIAL APPLICABILITY

The exhaust gas purifying system of the invention can reliably detectthe amount of accumulated particulates. When installed in a dieselengine vehicle, the exhaust gas purifying system can lengthen forcedregeneration intervals, and improve fuel efficiency.

1. An exhaust gas purifying system for an internal combustion engine,comprising: an exhaust-after-treatment device disposed in an exhaustsystem of the internal combustion engine, and including a particulatefilter configured to collect particulates from exhaust gas, and an NO₂generating unit upstream of or in the particulate filter; an excess airratio frequency calculating unit configured to calculate an excess airratio frequency at which an excess air ratio is equal to or less than apredetermined value during the operation of the internal combustionengine; a discharged particulate amount calculating unit configured tocalculate an amount of discharged particulates on the basis of an excessair ratio frequency at which an excess air ratio is equal or less than apredetermined excess air ratio; a filter temperature frequencycalculating unit configured to calculate a filter temperature frequencyat which the temperature of exhaust gas in front of the particulatefilter or the temperature of the particulate filter is equal to higherthan a predetermined value; a burnt particulate amount calculating unitconfigured to calculate an amount of burnt particulates on the basis ofthe filter temperature frequency; and a particulate accumulation amountcalculating unit configured to calculate an amount of particulates onthe particulate filter on the basis of the calculated amount ofdischarged particulates and the calculated amount of burnt particulates.2. The exhaust gas purifying system of claim 1, wherein: the dischargedparticulate amount calculating unit calculates an amount of particulatesdischarged in a given time period during which the excess air ratio isequal to or less than the predetermined value; the burnt particulateamount calculating unit includes a burning velocity calculating sectionwhich calculates a velocity for burning particulates on the particulatefilter on the basis of the filter temperature frequency, and derives anamount of particulates burnt in the given time period on the basis ofthe particulate burning velocity in the given time period and the amountof particulates accumulated in the given time period; and theparticulate accumulation amount calculating unit calculates an amount ofcurrently accumulated particulates on the basis of the amount ofpreviously accumulated particulates, the amount of particulatesdischarged during the given time period, and the amount of particulatesburnt in the given time period.
 3. The exhaust gas purifying system ofclaim 1, wherein the calculation of the amount of dischargedparticulates includes: downloading data on an amount of intake air anddata on an amount of injected fuel: calculating an excess air ratio λ ina given time period Δt on the basis of the amount of intake air and theamount of injected fuel; calculating an excess air ratio frequency γΔt,in which the excess air ratio λ is equal to or less than thepredetermined value in the given time period Δt, and calculating theamount of discharged particulates MeΔt {=fλΔt}, the foregoing proceduresbeing conducted in the named order.
 4. The exhaust gas purifying systemof claim 1, wherein the calculation of the amount of burnt particulatesincludes: downloading the catalyst temperature gt; calculating a filtertemperature frequency βΔt in the given time period Δt on the basis ofthe catalyst temperature gt; correcting the filter temperature frequencyβΔt using a correction factor K which depends upon an index NOx/Sootrepresenting that components of exhaust gas are suitable for burningparticulates; calculating a particulate burning velocity coefficient αΔt{=f(βΔt )} for the given time period Δt; and calculating an amount MbΔt{αΔt×PM_(i−1)} of burnt particulates on the basis of an amount PM_(i−1)of previously accumulated particulates and the particulate burningvelocity coefficient αΔt, the foregoing procedures being conducted inthe named order.